1. Field of the Invention
The present invention relates to a light scattering type smoke detector for detecting smoke by detecting a light scattered by smoke. More specifically, the present invention relates to a light scattering type smoke detector having a labyrinth member structure in which the capability for detecting light scattered by the smoke is enhanced by preventing the dispersion at the 0 point level of a detection output for each detector or by setting the 0 point to a low level.
2. Description of the Related Art
In general, as shown in FIGS. 5-7, this kind of the light scattering type smoke detector is provided with labyrinth members 9 for forming a smoke detecting chamber at the central portion of the detector so as to enable the easy inflow of smoke from the outside and the shut off light therefrom. Further, a light emitting device 12 and a light receiving device 13 are disposed so that the optical axes thereof intersect in the smoke detecting chamber formed by the labyrinth members 9. When smoke flows into the smoke detecting chamber, a light emitted from the light emitting device 12 is irregularly reflected by the particles of the smoke and the smoke can be detected by detecting a light scattered by the smoke.
This conventional smoke detector is arranged as described below. A smoke detecting unit main body 2 is formed to a substantially cylindrical shape as shown in FIG. 6. Also, as illustrated in FIG. 5, the smoke-detecting unit main body 2 is fitted in a recessed portion 1a formed at a lower central portion of a casing 1 and fixed to brackets 4 disposed at the rear of the casing 1 by screws 3. Note, the smoke detector shown in FIG. 6 is vertically upset with respect to the state in which it is actually mounted. The upper wall 8 is formed so that a plurality of labyrinth members 9 stand thereon. Then, the smoke detecting chamber is formed in the region surrounded by the labyrinth members. Each of the labyrinth members 9 has a horizontal cross section substantially formed to a ridge-roofshape or L-shape so that smoke can easily flow thereinto from the outside and light from the outside is shut off. Smoke inflow ports formed between the labyrinth members are surrounded by an insect control net 5 to prevent insects entering the smoke detecting chamber from scattering a light. A printed circuit board 30 with detector circuitry packaged thereon is placed on a upper wall 8. Upper side of the printed circuit board 30 is covered by an upper seal cover 20. An outer cover member 7 is fitted to the lower side of the upper wall 8 through a lower wall 6 interposed therebetween. The outer cover member 7 has a plurality of openings 7a for allowing smoke to enter the smoke-detecting unit 2.
The upper wall 8 is also provided with recessed portions 10, 11 and a shade plate 14. The light emitting device 12 and light receiving device 13 are accommodated by the recessed portions 10, 11, respectively so that the respective optical axes thereof intersect at the center of the smoke detecting chamber formed by the labyrinth members 9. Further, the shade plate 14 is standingly formed to prevent a light emitted from the light emitting device 12 from directly reaching the light receiving device 13. Further, an opening is formed to each of the recessed portions 10, 11 so that the light from the light emitting device 12 is not directly received by the light receiving device 13.
In the detector arranged as described above, a detection output from the light receiving device 13 is set to a 0 point level, namely the state of no smoke. On the other hand, when smoke flows into the smoke detecting chamber, a scattered light is produced by the particles of the smoke and the light is detected by the light receiving device 13. More specifically, an amount of light in the light receiving device 13 is increased with respect to the aforesaid 0 point level, whereby it is detected that the smoke flows into the smoke detecting chamber and thus the smoke is detected.
Incidentally, in the detector arranged as described above, the light from the light emitting device 12 must not be incident upon the light receiving device 13 as far as possible when there is no smoke to improve the sensibility of the detector. For this purpose, this detector is designed to introduce the light from the light emitting device 12 to the outside of the smoke detecting chamber through the labyrinth members 9. Nevertheless, the light is partly incident upon the light receiving device 13 after it has been reflected by the labyrinth members a plurality of times. Therefore, since this light acts as a noise light, a detection signal is produced by the light receiving device 13 even if there is no smoke in the smoke detecting chamber. That is, a so-called 0 point level exists in the detector due to a detection signal produced by the light receiving device 13 under the influence of the noise light. Therefore, in the light scattering type smoke detector, the 0 point level is set first and smoke is detected by using the 0 point level as a reference.
Therefore, the structure inside the detector must be arranged so as to set the 0 point level as low as possible by reducing a noise component to improve the sensitivity of the detector. That is, the interior of the detector is preferably formed to have such a structure that the light from the light emitting device 12 is not reflected in the direction of the light receiving device 13. For this purpose, the labyrinth members of the conventional detector are formed so that the edges thereof have a radius of curvature as small as possible, i.e., r=0 is achieved. Note, the labyrinth members are molded of plastics by using, for example, a metal mold.
Although the aforesaid detector is arranged so that the light from the light emitting device 12 is not directly received by the light receiving device 13, however, the labyrinth members 9 are disposed around the light receiving device 13 and thus the reflection of light therefrom cannot be avoided. Further, although the edges of the labyrinth members 9 are also formed to have the radius of curvature as small as possible as described above, the light from the light emitting device 12 is also irregularly reflected by the edges of the labyrinth members 9 as shown by the arrows in FIG. 7. Therefore, the 0 point level of the detection output is increased by the irregular reflection.
On the other hand, although the light emitting device 12 is usually composed of a near infrared LED or the like, it has a dispersion in the direction of a light emitting optical axis due to manufacture assembly and the like. Further, although the LED is fixed in the recessed portion 10 and the light emitting range thereof is restricted by the opening of the recessed portion, a dispersion is caused to the light emitting range due to the dimensional errors of the LED and the recessed portion 10 and the assembling error of the LED. More specifically, when the LED has a light emitting range of 10-20.degree., a dispersion of +/-3-5.degree. is caused.
As described above, in the conventional detector, the number of the edges of the labyrinth members 9 located in the light emitting range and the positions thereof are different to each detector due to the dispersion of the light emitting range of the LED. Therefore, a problem arises in that a large dispersion at the 0 point level of a detection output is caused to each detector.
Further, in the above conventional detector, since the labyrinth members 9 are made so that the edges thereof satisfy r=0, the edge portion of a metal mold is arranged to achieve r=0. Consequently, the curvature of the edge is different due to the flow of plastics and thus a dispersion of, for example, 0.03&lt;r&lt;0.08 is caused as shown in FIG. 8. Further, the edges of the labyrinth members 9 may be chipped off when they are released from the metal mold. Therefore, a problem arises in that a large dispersion at the 0 point level of the detection output is caused to each detector due to the dispersion of the radius of curvature of the edges and the chipping-off thereof. Further, a problem also arises in that defective products are made and yield deteriorates depending upon said manufacturing process.